Oriented solidification of metals and of semiconductors may be performed on Earth or in space under microgravity.
The crucibles used to contain the materials to be melted, in particular the crucibles used in space under weightlessness, have a two-layer wall, namely an outer metal envelope providing mechanical strength, sealing, and protection against crucible defects, and a crucible per se which contains the material to be melted and which has physical and chemical characteristics which are compatible with the characteristics of the molten sample, but which has no guaranteed mechanical strength nor thermal conductivity matching, thus inducing a deformation of the heat flow lines.
The crucible per se is made from a crucible optimized material which is the best for a given sample. For example, pyrolytic boron nitride for gallium arsenide, quartz for germanium, alumina for indium antimonide. These crucible optimized materials have been selected because they give the lowest sample contamination. Unfortunately, since they are generally ceramics and/or glasses, they are brittle and prone to fracture propagation.
The presence of an outer metal envelope for the purpose of providing protection against the escape of vapor in the event of the internal crucible rupturing, contributes to deforming the heat flow lines in the sample, thereby degrading the quality of the resulting single crystal. Further, even using a double wall does not guarantee adequate mechanical strength and reliability at very high temperatures because of problems of expansion and the risk of the outer metal envelope being corroded by the sample to be melted which is often highly corrosive, particularly if it is a semiconductor. Further, some 3-5 type semiconductors (such as gallium arsenide) or 2-6 type semiconductors include components which are very toxic and which require that the entire cartridge structure should be reliable, which is difficult if the cartridge is itself constituted by a plurality of successive walls.
The present invention seeks to remedy the above-mentioned drawbacks and to provide, in particular, improved control of the melt front, thereby enabling better quality single crystals to be obtained while ensuring improved mechanical strength and reliability when hot over prior art cartridges.